URGENT:Can I fill the pylons with"great stuff"foam and cut??

I'm in progress of taking a 1979 Hobie and and 1989 Hobie and making one good boat. I took apart the 89 and found this. All the pylons are a hair over 5" except for one that is 6.5". Not sure why, but it might work out perfectly. The one that is 6.5" is also somewhat corroded/chipped at the top so if I can cut 1.5" off the top it will be perfect. Can anyone think of why cutting it down would be a bad idea? Ill also have to drill out the holes on the trampoline assembly because they are a little bit wider then what came on the 1979. Also, one hull from the 1989 has no foam in the pylons. Can I fill them with just the "great stuff" foam in a can? Just to make them water right? Any other suggestions while I have the boat all apart?

I dont' see why it would hurt anything to cut that one down if it's seated good at the bottom. There is a flat structural piece that it sits on at it's bottom. If it's down good on that, it should be fine to cut it. I don't remember the measurement of amount of pylon sticking up.???. Originally, there is just an expanded polyethyelene foam plug stuck down in there to seal it. The Great Stuff will work but I don't remember where the breather hole is on a 16. There may be a plastic straw stuck down through the front pylon foam block. They have to have a breather hole somewhere.

It looks like the one "long" pylon has been repaired- there is a weld bead all around it. The previous owner probably cut off the bad section of pylon and welded on a new section but forgot to trim the pylon to the proper length afterwards.

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...and it would appear that the corner casting would then have to slip over the weld bead, ending almost on the bead? Might increase point loads at the repair area.

Probably would have to grind more of the bead away (looks like it was on one side but not the other).

My gut feeling is saying that the repair has "worked" for this long....might want to leave well enough alone or wait until new damage / stress appears. Just my 2 cents.

Another idea that popped into my head (no idea if this is a good idea or not, though...) would be to dig out as much foam as you can and fill the pylon with long strand reinforced fiberglass filler (heavy but reasonably strong). This might allow you to cut / grind the pylon to your desired length without making it any weaker than it is now, and might be stronger.

Is it actually a weld though? It looks like it, but could it also some weird looking corrosion? Those pylons are pretty rough, fresh water rinsing was definetly not the norm for that boat.

Yeah its definitely a weld. More aluminum was attached on the inside for extra reinforcement it seems. I think I will will this with foam and then cut it to size. Hopefully the casting will fit over the weld and everything will be fine. The old trampoline assembly that was on there was super loose with a bunch (like 30) rivets falling out of it when I took it apart. I think it may be because the trampoline was put on there somewhat distorted with this abnormal hull pylon. My question is, what is the vent supposed to be like and where does it go? The other hull was also welded but filled with foam and I do not see a vent there either. I am going to buy a closed cell foam to pour in here, does anyone have instructions on how to do this?

I don't see how the corner casting is going to fit over that weld unless it is ground down. That may be why the previous owner left the repair a little long.

I wouldn't be overly concerned about the vent tube. As long as you keep the plugs out when you're not using the boat you should be fine.

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Really you think the casting has that snug of a fit on there? I guess I will have to give it a try when I get back home and see if it would fit,,

Definitely. The casting has a consistent profile over the whole length of the pylon and it is (or at least should be) a snug fit to begin with - they often have to be installed using a mallet to knock the casting onto the pylon.

I would also be concerned that the location of the existing pylon bolt hole is going to interfere with the new bolt hole that you'll have to drill.

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I was thinking about that too. Especially since the hole that was there already is kind of elongated. Ill measure a few things next time I am where the boat is and then decided on what to do. Worst comes to worst I can cut the pylon off my old hulls and have it welded on this one again.

I wouldn't do that. Welding aluminum extrusions reduces the strength in the weld area by as much as 40%. Your pylon is already weakened by being welded.

You're better off filling the holes with thickened epoxy / fiberglass or you could try brazing them closed with something like Alumaweld. After cleaning them up really well with sandpaper, put tape over the holes on the outside and gob the material into the holes and surrounding area on the inside.

The bolt holes don't take much stress - the bolts are there just to keep the hulls and tramp frame together. It's when everything is loose that the holes get elongated. Another good reason to have a tight trampoline.

What does it take to just remove the whole pylon and replace it? Its a different extrusion now, but didn't it used to be just the mast extrusion?

yee-haw! that'll keep him busy for a while. Pylon is sunk in to a foam shoe and the whole assembly glassed to all adjacent surfaces.

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It's actually the same extrusion as the pre-1995 rear crossbar (before integrated traveller tracks) and a Hobie 14's mast (well, it used to be before they started using the special "no rear slot" extrusion).

The pylon is inserted as part of the deck layup - using lots of glue. The pylon shoe is attached to the bottom end - using lots of glue. When the hull and deck are joined, the shoe is seated against the hull with lots of resin-saturated fiberglass.

It would be very, very difficult to remove the pylon. You certainly couldn't do it in one piece without damaging the surrounding structure - and it would involve cutting a deck port in behind it to access the lower end. I suppose putting in a new one would be comparatively easy once you got the hole cleaned up.

Yeah I definitely do not plan on removing this pylon. If I were to fill this with foam, will any leak out the bottom into the hull? Water gets into the hull from there but it seems like the opening down there isnt large enough to let too much foam get through. What would be the best way to fill it with foam so it doesnt expand too much and start breaking things? The plan is to fill with 2# foam from the link listed below, and then cut as much off the top as I can while having the casting not interfere with the weld. Hopefully take 1.5 inches off so it is the same height as the others. Do I need to worry about a vent? I also would like to find a stainless steel sleeve to put inside the bolt hole/foam so that bolt isnt rubbing against the foam in that elongated hole.

http://uscomposites.com/foam.html